The Effect of Air Movement, Air Temperature and Infrared Radiation on the Energy Requirements of Sheep.

In Britain sheep are normally kept out-of-doors in winter and on hill farms they may be exposed to very severe weather conditions. On some hill farms pregnant ewes weighing 100 lb at the beginning of winter may well weigh 80 lb just before they lamb if the intervening winter weather has been severe. Deaths of ewes and more particularly of newborn Iambs can be very many in bad winters (Robinson, 1953). Part of the weight loss may be attributed to lack of food and many of the deaths to starvation, but an increase in the energy requirement of sheep due to the stresses imposed by cold, wind, rain and snow must also be considered. The stresses of cold and lack of food are usually conjoint, for ice and snow make it impossible for the hill sheep to gather food. Similar climatic stresses also occur in New Zealand. Many deaths occur when cold weather with wind follows a second shearing of sheep in winter time. Little is known of the increase in the energy expenditure of sheep that occurs in such adverse circumstances. Calorimetric experiments with adult sheep (Graham, Wainman, Blaxter & Armstrong, 1959; Blaxter, Graham & Wainman, 1959; Blaxter Graham, Wainman & Armstrong, 1959 ; Armstrong, Blaxter, Clapperton, Graham & Wainman, 1960) have shown that at maintenance levels of feeding the critical temperature of sheep, defined as the air temperature below which heat production increases, falls from about 30' for sheep with 2 mm fleece to 20' for those with 18 mm fleece, to 10' for those with 35 mm fleece and to about -3' for those with 100 mm fleece. The increase in energy requirement in environments with temperatures below these critical temperatures can be predicted. These critical temperatures, however, refer to the environmental conditions in the instrument used to measure them, namely a low air velocity and a higher incoming i.r. radiation than is common out-ofdoors. Golding (1961) has recently pointed out that in west Scotland the annual mean wind speed is 12.5-17.5 miles/h and that on hill sites on Harris and on Islay the mean annual wind speed is 27 miles/h. It is certain that the estimates of critical temperatures made in calorimeters in which air velocity is about 0-4-0-6 miles/h are gross underestimates when winds of this magnitude are encountered. The experiments reported below were undertaken to examine the effects of wind velocity on the energy expenditure of sheep under controlled conditions and to find how long it takes sheep to reach constant heat production after changes in air temperature, air velocity and incoming long-wave radiation intensity. All these experiments were regarded as essential preliminaries to studies planned to take place in